Pokemon Plus and Pokemon Minus, like
>>22224527 said.
Plot revolves around Team Matrix (or something), an evil syndicate specializing in cybercrime, especially stealing pokemon out of boxes. They're fascinated with Porygon, the only artificially created, digital pokemon, and are determined to make a second one, only much, much stronger. But their leader got arrogant, and faults in the code snowballed, and when it was too late to stop it, they gave life to a half-formed digital pokemon, identified as: MissingNo.
Literally, MissingNo. is made a pokemon, sort of. A glitchy spectre of forms in the rough shape of his most infamous incarnation, pic related. MissingNo.'s errors are causing it to destabilize all technology around it, but its researchers hypothesize that it could be stabilized if contained in a Pokeball. So the player and the team leader race off to try and catch MissingNo. first.
A recurring theme with Team Matrix is that they always seem one step ahead of you or are constantly ambushing you - in the final battle, it's revealed that an Aide to the Professor that gave you your Pokedex was a mole, and your Pokedex has been bugged with trackers - just like every single other one in the region, a key tactic that assisted Team Matrix in accessing and stealing boxed Pokemon data.
